1. Field of the Invention
The invention relates to an electrostatic chuck for retaining a workpiece, particularly, such a chuck having a grooved insulating layer which supports the workpiece, and a method of fabricating this chuck.
2. Description of the Background Art
In general, chucks are used in various material processing systems to retain workpieces, such as semiconductor wafers or dielectric substrates, in a mechanically stationary position while such a system processes the workpiece. In particular, during semiconductor wafer processing, chucks are used to position a substrate of semiconductor material within a vacuum chamber while the substrate undergoes one or more process steps for creating integrated circuits on the substrate. As is well known in the art, such process steps include, for example, ion implantation, sputtering, plasma etching, or chemical vapor deposition.
Generally, during processing, the semiconductor wafer is repeatedly heated and cooled while undergoing various process steps. Typically, all of the process steps are accomplished in a vacuum chamber having a relatively high vacuum. Because a vacuum does not provide heat conduction or convection, a vacuum environment provides limited heat transfer from the wafer. Conventionally, to improve heat conduction from the wafer, the wafer rests upon a massive, heat conductive pedestal, typically formed of a metallic material. Additionally, the periphery of the wafer may be mechanically clamped to the pedestal. One or more grooves are often formed in the surface of the pedestal which faces the wafer. These grooves are typically produced in a defined pattern by appropriately machining the surface of the pedestal. To provide a spatially uniform conductive heat transfer from the wafer to the pedestal, a heat transfer medium, typically a gas such as helium, is pumped into the grooves. These grooves act as flow conduits which uniformly distribute the heat transfer medium along the surface of the wafer that faces the pedestal. Such a conductive heat transfer process is commonly referred to as a "gas assisted heat transfer". Such gas assisted heat transfer is disclosed by Tsui in European published patent application Ser. No. 0,397,315, and which is incorporated herein by reference.
Gas assisted heat transfer between a workpiece and a pedestal usually relies upon equipment which mechanically clamps a wafer to a pedestal, i.e., a mechanical chuck. Such a mechanical chuck is disclosed by Morley in U.S. Pat. No. 4,603,466, and which is incorporated herein by reference. Specifically, this chuck mechanically clamps a wafer about its periphery to a dome shaped plate (pedestal). Ports are provided through the plate to introduce a gas to an interstitial space between the pedestal and the wafer. A machined annular groove extends about the periphery of the pedestal surface. This groove is connected to a vacuum pump to remove the gas from the interstitial space to minimize possible gas leakage from the interstitial space into the vacuum environment in which the wafer is contained.
In an effort to produce equipment which can retain a workpiece without grasping or clamping the workpiece, i.e., without using a mechanical chuck, the art has developed so-called electrostatic chucks. An example of an electrostatic chuck is disclosed by Briglia in U.S. Pat. No. 4,184,188, which is also incorporated herein by reference. An electrostatic chuck retains a workpiece, such as a semiconductor wafer, by generating a charge differential between a surface of the wafer and one or more electrodes located within the body of the chuck. The ensuing electrostatic force developed between the wafer and the electrodes retains the wafer against the chuck body. The electrodes are typically insulated from the wafer by a relatively thin layer or film of insulating or dielectric material (hereinafter the dielectric layer). As such, the dielectric layer provides a surface upon which the electrostatic force retains the wafer. There are many well known techniques for generating the electrostatic force in an electrostatic chuck, all of which are not relevant here.
For various reasons, including facilitating easy removal of the wafer from the chuck body after processing, the surface of the dielectric layer may be molded or shaped. In particular, as described in Japanese Laid-Open patent application number 60-261377 published on Dec. 24, 1985, the surface of the dielectric layer against which the wafer is retained may contain an embossed pattern of protrusions which support the wafer. Alternatively, as described in Japanese Laid-Open patent application number 63-194345 published on Aug. 11, 1988, strips of a rubber material may be positioned between the dielectric layer and the wafer to provide spatially periodic wafer support.
As with the mechanical chuck, pumping a heat conductive medium between the wafer and the electrostatic chuck would facilitate improved heat conduction from the wafer to the chuck. However, in the past, no economical techniques were available for producing grooves in a dielectric layer. As a result, electrostatic chucks generally do not have gas assisted cooling. In particular, neither of the periodic support structures described in Japanese Laid-Open application numbers 63-194345 and 60-261377 and discussed above contains channels or grooves through which a heat transfer medium could be pumped. Additionally, as discussed below, various methods have been used in an attempt to produce grooves in the surface of a chuck body. However, such methods have proved economically unfeasible.
One such technique for producing grooves in a dielectric surface involved machining the grooves into the surface. Unfortunately, this proved to be considerably difficult. In particular, when a relatively thin dielectric layer was machined to form such grooves, that layer tended to fracture or chip; thus, rendering the entire chuck body useless. Additionally, this machining was a relatively slow process that added excessive labor costs to the manufacturing cost of the chuck as well as resulted in low yields.
Another technique for producing grooves in an otherwise smooth surface of a dielectric layer use a die to emboss a groove pattern into the dielectric layer before hardening the layer. However, when an embossed layer, typically formed of a relatively thick layer of ceramic material, was hardened by firing, the ceramic material shrank in a non-uniform manner. As such, dimensions of an embossed pattern tended to change substantially, thereby producing what is commonly referred to as "dimensional shift". Dimensional shift often resulted in non-uniform grooves. Consequently, when a heat transfer medium was pumped into the grooves, the medium was non-uniformly distributed. As a result, a wafer used in conjunction with such a chuck tended to exhibit non-uniform cooling. This, in turn, unfortunately caused detrimental effects, such as wafer warping, during subsequent processing. Additionally, only a limited number of dielectric materials have the viscosity allowing them to be embossed. This, in turn, by limiting the selection of useful materials, restricted the range of dielectric coefficients that could be achieved. Therefore, embossing was not generally favored as a technique for producing such grooves in the surface of a chuck body.
Therefore, a need exists in the art for a practical method of producing an electrostatic chuck having heat transfer medium distribution grooves in a surface of a dielectric layer that forms a portion of the chuck. This method should produce grooves of substantially uniform dimensions. Advantageously, an electrostatic chuck formed by such a method would, when used during wafer processing, successfully facilitate use of gas assisted heat transfer.